Revival of the heterologous prime‐boost technique in COVID‐19: An outlook from the history of outbreaks

Abstract Background The heterologous prime‐boost vaccination technique is not novel as it has a history of deployment in previous outbreaks. Aim Hence, this narrative review aims to provide critical insight for reviving the heterologous prime‐boost immunization strategy for SARS‐CoV‐2 vaccination relative to a brief positive outlook on the mix‐dose approach deployed in previous and existing outbreaks (ie, Ebola virus disease (EVD), malaria, tuberculosis, hepatitis B, HIV and influenza virus). Methodology and Materials A Boolean search was carried out to find the literature in MEDLINE‐PubMed, Clinicaltrials, and Cochrane Central Register of Controlled Trials databases up till December 22, 2021, using the specific keywords that include “SARS‐CoV2”, “COVID‐19”, “Ebola,” “Malaria,” “Tuberculosis,” “Human Immunodeficiency Virus,” “Hepatitis B,” “Influenza,” “Mix and match,” “Heterologous prime‐boost,” with interposition of “OR” and “AND.” Full text of all the related articles in English language with supplementary appendix was retrieved. In addition, the full text of relevant cross‐references was also retrieved. Results Therefore, as generally evident by the primary outcomes, that is, safety, reactogenicity, and immunogenicity reported and updated by preclinical and clinical studies for addressing previous and existing outbreaks so far, including COVID‐19, it is understood that heterologous prime‐boost immunization has been proven successful for eliciting a more efficacious immune response as of yet in comparison to the traditional homologous prime‐boost immunization regimen. Discussion Accordingly, with increasing cases of COVID‐19, many countries such as Germany, Pakistan, Canada, Thailand, and the United Kingdom have started administering the heterologous vaccination as the technique aids to rationalize the usage of the available vaccines to aid in the global race to vaccinate majority to curb the spread of COVID‐19 efficiently. However, the article emphasizes the need for more large controlled trials considering demographic details of vaccine recipients, which would play an essential role in clearing the mistrust about safety concerns to pace up the acceptance of the technique across the globe. Conclusion Consequently, by combatting the back‐to‐back waves of COVID‐19 and other challenging variants of concerns, including Omicron, the discussed approach can also help in addressing the expected evolution of priority outbreaks as coined by WHO, that is, “Disease X” in 2018 with competency, which according to WHO can turn into the “next pandemic” or the “next public health emergency,” thus would eventually lead to eradicating the risk of “population crisis.”

The best identified and cost-effective strategy to prevent infectious diseases is via safe vaccination dosing. 2 The essential goal of vaccination is to generate potent and long-term protection against diseases. Thus, to combat the deadly consequence of the COVID-19 outbreak, scientists accelerated the global vaccine development for SARS-CoV-2. Vaccines against COVID-19 are available in variety and prepared using conventional techniques using inactivated or live attenuated viruses. Other approaches utilize recombinant proteins and virtual vector-based vaccines. 3 The concerns regarding traditional vaccination include safety issues, undesirable host reactions, and problematic culturing of dangerous microorganisms that prompted the development of subunit vaccines. 4,5 However, recombinant subunit vaccines are weakly immunogenic and necessitate additional components to improve protective immunity. Thus, it is recommended to enhance the effectiveness of subunit vaccines through a prime-boosting immunization strategy. 4 A prime-boost vaccination strategy is an immunization program using the identical immunogen during the primary and booster doses (Homologous prime-boost immunization) or using a different immunogen (Heterologous prime-boost immunization) to boost the immunogenic priming regimen. 6 The majority of existing immunization regimens incorporate a second homologous booster dose after a month of the prime-dosing regimen. However, comparatively, the more efficacious, heterologous prime-boost technique is also considered a choice of booster regimens in various previous and existing outbreaks, that is, Ebola virus disease (EVD), malaria, tuberculosis, hepatitis B, HIV, and influenza virus. Furthermore, even in the existing pandemic of COVID-19, many countries such as Germany, Pakistan, Canada, Thailand, and the United Kingdom have started administering the mix-and-match technique. Thus, it rationalizes using the available vaccines to assist in the global race to vaccinate the majority to curb the spread of COVID-19 efficiently.
Besides, there are safety concerns regarding the mix-and-match technique due to insignificant large-controlled trials. Thus, there is no particular guideline or approval from the World Health Organization (WHO) regarding the matter. Therefore, our review aims to advance the understanding of the heterologous prime-boost immunization strategy relative to a brief positive outlook of trials based on its application in dealing with the outbreaks as mentioned earlier. We also append our article with a set of recommendations for making its revival possible in combatting COVID-19.

| METHODS
A Boolean search was carried out to find the literature in MEDLINE-PubMed, Clinicaltrials, and Cochrane Central Register of Controlled Trials databases up till December 22, 2021, using specific keywords that include "SARS-CoV2," "COVID-19," "Ebola," "Malaria," "Tuberculosis," "Human Immunodeficiency Virus," "Hepatitis B," "Influenza," "Mix and match," "Heterologous prime-boost," with interposition of "OR" and clinical trial registries and preprint repositories were included to provide a more general report of serologic data reported to date for the mixeddosed approach to add to the literature. However, all the reviews, editorials, commentaries, case reports, case series were excluded.

| The mix-and-match technique
To begin with, for vaccines to be effective, it usually requires multiple vaccine doses, that is, diphtheria, tetanus, and pertussis (DTP) vaccine.
Further, multiple immunizations can be given in either traditional homologous or heterologous prime-boost format to make the existing vaccines successful. In particular, the concept of the heterologous prime-boost technique, also known as the mix-and-match technique, started budding in the 1990s when HIV researchers tested this strategy. The trials were conducted to analyze the technique because the traditional approach for vaccine delivery was incapable of eliciting the complex immunological mechanisms needed for potential protection from HIV infection. 7 The strategies practiced so far involve prime dosing of the homologous or the evolving heterologous vaccine regimen followed by a similar or a mix-and-match booster, even resulting in a 3-dose regimen. 8,9 Regarding the remarkable report of primary outcomes, that is, safety, reactogenicity, and immunogenicity quoted by the pre-clinical and clinical studies, it is suggestive that the mix-and-match procedures employ diverse modes of gene expression that tend to be more immunogenic than the traditional one. 10 Furthermore, despite the robust immunity offered by it, the mix-  Table 1.

| An outlook from the history of outbreaks
The heterologous prime-boost technique, which aims to increase the protective efficacy and rationalize the usage of the available vaccines, can also elicit a more robust and long-lasting immune response as compared with the single vaccine or traditional homologous primeboost immunization regimen. It is made evident by the mixed-dose approach explicitly adopted in Phase I and II trials quoting excellent safety and improved immunogenicity by the Ebola vaccine developed by Johnson & Johnson. 19 Notably, the first shot of the Ebola vaccine is composed of the same adenovirus as the AstraZeneca vaccine, and the second shot of an MVA vector (a modified version of a poxvirus: a type that is also under investigation for future Covid-19 vaccines).
Similarly, a pneumococcal conjugate vaccine dose is followed by a booster containing a pneumococcal polysaccharide to trigger more powerful protection in the elderly. 20  Alternatively, considering BCG as a boost following a DNA vaccine prime, as in one study conducted in calves, DNA prime with Ag85B, MPT64, and MPT83 antigens followed by a BCG boost was able to elicit higher immune responses and better protection than BCG alone against mycobacterium Bovis challenge. 25 Additionally, several previous studies also identify the relationship between mixand-match vaccine and age; that is, the beneficial effect is more pronounced when given in early life, as evident in a study about heterologous effects of BCG. 26 However, another study suggests that early T A B L E 1 Heterologous prime-boost technique vs homologous prime-boost technique (a general comparison)

Advantages Disadvantages
• The mix-dosing approach provides a wide range of cellular immune responses quantitatively and qualitatively. 12,13 • The heterologous prime-boost technique may report more short-term side-effects, that is, fever and fatigue, than homologous booster after priming with a dose of homologous vaccine. 11 • It can have significant practical applications as in the current global pandemic of COVID-19 by compensating the scarcity of vaccines supply. 14 • It may also result in worse immune responses in younger individuals as they tend to have a more active immune system than vaccine recipients over the age of 50. 11 • It can expand the scope of immune responses, 15 resulting in a shift in the immunodominance hierarchy. 16 • The prime-boost vaccine approach can also improve the effectiveness of existing vaccines. 17 • The mixed-dosing approach further possesses potential in cancer immunotherapy. 16,18 immune response to monovalent influenza vaccine changes significantly after 35 years. 27 Conclusively, mix-and-match approaches generally have been successfully employed in addressing the majority of outbreaks to tackle many others, that is, seasonal influenza virus, [28][29][30] papillomavirus, 31

| Researchers and the battle of COVID-19
The decades-long-struggling in the HIV vaccine field has driven scien-  SARS-COV protein two after administering these heterogeneous shots to assess the potency of this method. These assessments included levels of anti-spike immunoglobulins using ELISA at baseline and 4 weeks postboost, and many others. (ISRCTN69254139, UK) An electronic diary card also measured incidences of serious adverse events.
However, the few limitations of the process mentioned above included exceptional disruption in vaccine distribution. People could not get the second dose after their first one, and some vaccines were unavailable for emergency use. In contrast, some vaccines might not have the same technological platform, so mixing was impossible. Further, not enough trials are available to prove anything as of yet. 20 Therefore, since it is unchartered territory, governments should be meticulous in keeping control of only mixing vaccines in trial studies and not in the general public due to a lack of evidence for the established efficacy of these trials.

| Challenges: The grey area
More importantly, many previous studies have postulated that this approach of mix-and-match results in much better protection with an increase in antibody titer compared with those with homologous or the same vaccine approach (Tables 2 and 3)

| RECOMMENDATION AND THE FUTURE INSIGHTS
The heterologous prime-boost technique is not something novel as provided by the outlook of trials from the history of outbreaks earlier (Table 2) and comparatively proved to be a more effective technique than a homologous one. The mix-and-match approach also compensates for the scarce vaccine supplies and unequal roll-out of COVID-19 vaccines. Therefore, it paves the way for the method to be further trialed in existing COVID-19 vaccines (proven safe by WHO) on an immediate and large-scale basis.
However, due to the safety concerns, the sample size for trials is an area of concern. Thus, incentives such as awareness campaigns and commercials focusing not only on clearing the mistrust regarding safety concerns of the technique but highlighting the long-lasting immunologic benefits from getting the mix-dosed booster jab would encourage the attention of more volunteers. Subsequently, in the long run, the considerable sample size of the trials will provide strong evidence to support the relevance of the heterologous prime-boost technique. Further, considering parameters like pharmaceutical aspects of vaccines and clinical aspects and high demographic variance while conducting trials would aid in providing a more comprehensive analysis of the reactogenicity of the technique. 11,25 Moreover, making the availability of more preprint data can make meta-analyses possible, which would also aid in overcoming the barrier of sample size.
Consequently, with the whole world getting on the same page by prioritizing the technique to vaccinate their citizens, it would act as a more prompt attempt than other preventive measures (ie, wearing masks and strict lock-downs) to curtail the pace of mutation of COVID-19 effectively and efficiently. Additionally, we can further expand the scope of immune responses by addressing the current scenario timely with serious consideration of reviving the heterologous prime-boost technique. 15 Thus, eventually can prepare researchers to devise countermeasures ahead of time to combat other priority diseases 71 coined by WHO, that is, "Disease X" in 2018 and different much challenging variants of concerns, including Omicron, competently.

| CONCLUSION
Conclusively, the mix-and-match technique provides more longlasting immunological and logistical benefits than the traditional one.
Additionally, providing a brief outlook in dealing with previous and existing outbreaks in the article promotes the revival of the technique in combatting the COVID-19. However, further studies evaluating the strategy incorporating vaccines, such as Moderna, Pfizer, AstraZeneca, and Covax, are ongoing and are also crucial to informing the appropriateness of heterologous prime-boost schedules. Consequently, the future insight, after approval of this technique across the world, would result in healthcare policy makers to competently preclude the evolution of other priority outbreaks according to WHO, in turning into the "next pandemic" or "next public health emergency" to eventually eradicate the risk of "population crisis." FUNDING None declared.

CONFLICT OF INTEREST
The authors declare no conflicts of interest.